Ignition magneto



June 8 1926. 1,587,958

J. C. HUTTON ET AL IGNITION MAGNETO Filed Nov. 25, 1924 Patented June 8,1926.

UNITED STATES PATENT OFFICE.

JAMES COCI'IRANE HUTTON, 0F BIRMINGHAM,

AND HAROLD ERNEST WHITEHOUSE,

0F COVENTRY, ENGLAND, ASSIGNORS TO THE BRITISH LIGHTING AND IGNITIONCOMPANY LIMITED, OF B. L. I. C. ORKS, ASTON, BIRMINGHAM, ENGLAND.

IGNITION MAGNETO.

Application filed November 25, 1924, Serial No.

This invention relates to ignition magnetos for internal combustionengines, said magnetos being of the type in which either the armature isfixed there is employed in conjunction therewith either a rotary magnetor rotating inductors and s ationary magnets, or the armature isrotatable and the magnets are stationary.

lVith magnetos as hereinbefore constructed their sparkingcharacteristics at slow speeds are much worse in the retarded positionof the timing than in the advanced position; this is due to the factthat at the moment wien the contacts open the length of the air gapbetween the rotor and stator pole tips is much greater in the retardedposition than in the advanced position, With the result that a largeproportion of the flux has already reversed its direction in the coil.core.

In order to improve the performance of magnetos for the retardedsparking position various means have been proposed, one of the mosteffective being a steel sleeve positioned between the stator and rotorshoes, said sleeve being arranged to rotate with the timing lever so asto maintain an air gap of unvarying size between the edges of the rotorand stator poles with either an advanced or retarded position of timing;this method however has the disadvantage that an extra air gap isintroduced between the rotor and stator and moreover the construction ofmagnetos with this arrangement is expensive.

Another method that has been proposed 1s to provide the stator poleshoes with ordinary extended or over-hanging tips extending along eithera portion or the whole length of the pole shoe; with this arrange meatwhen the machine is working at low speeds the performance for theretarded sparking position is improved. but the performance in theadvanced sparking position is made worse owing to the overlapping tipspreventing a sudden complete reversal of the flux in the coil core whenthe contacts are opened.

A modification of this method which has seen suggested is to provideextended tips which however were separated from the main stator poleshoes by a short air gap.

but this construction though effective-in,

volves certain mechanical difliculties.

752,151, and in Great Britain February 15, 1924.

Lastly it has also been proposed to separate the ordinary extended tipsfrom the main portions of the stator pole shoes by a narrow bridge ofrelatively high magnetic reluctance; this method while producing someimprovement is not so effective as the last mentioned method, since onaccount of mechanical considerations the bridge cannot be made as smallas'is desirable to obtain the best electrical working of the magneto.

The magnetos to which alone the present invention has reference are ofwherein the faces of the two stator pole shoes, as also the faces of thetwo rotor pole shoes, are symmetrical, and in which the fully advancedtiming position of the rotor corresponds to the maximum sparkingposition and the fully retarded timing position of the rotor correspondsto the minimum sparking position, there being a gradual decline in theperformance of the magneto between the maximum and minimum sparkingpositions, and the object of the present invention is to overcome thedifficulties above mentioned with magnetos functioning in this mannerand to improve the slow speed performance of the magneto when thesparking position is retarded without detrimentally affecting itsperformance when the timing is in the advance position so as to obtainsuch an improved performance in the retarded position that efficientworking of the magneto can be ob tained with a large angle of retard.whereby good. sparking characteristics of the mag neto can be securedwith very slow engine speeds. For this purpose, according to the presentinvention each of the pole shoes of either the-rotary member or thestationary member of an ignition magneto of the kind hereinbeforereferred to is provided with an extension piece or tip connectedthereith by a bridge member of low magnetic reluctance so that the saidextension piece is separated from the main body of the pole shoe by arecess, whilst each of the corresponding pole shoes of the other memberis formed With a recess or slot therein separating the main body of thepole shoe from the end or tiplike portion thereof, the totalcircumferential length (that is measured in the planeof rotation) of theextension piece and recess of each pole shoe of one member the typebeing substantially equal to that of the tip and recess ot-eachpole shoeof the con responding member, and the arrangement being such that l'orthe advanced tuning positionoli therotor the extension pieces of thepole shoes of one nieinberlie opposite the rccessesor slots .Olthepoleshoes of the other member and are separated therefrom in a radial.direction by a comparatively large ailgap when the illlli break occurs,there'heing two points of liux break on each side of the rotor in thesaid advanced timing position, whilst tor the retarded tiining positionof the rotor there is only one point of llllli break on each side of therotor,-the said flu); break then taking place between .the extensionpieces of the pole shoes of one member and the oppositeend portions ortips on the pole shoes of the-other l'l'lOll'lbOl'. The extension piecesortips therefor-e exercise the desired eliect by brido'ing across theair gap between the stator and rotor pole shoes in the re tardedv timingposition otthe rotor, the said extension piececor tips automaticallycoming into operation as hereinafter described when the timing oi themagneto is retarded. he arrangement according to the present inventionhas the advantages of lending itself to a robust mechanical constructionwhile avoiding; the use oil? any extra'inoving parts of the i'nagnetobeyond those ordinarily involved.

in order that the said invention may be clearly understood and readilycarried into effect, the same will now be described more fully withreference to the accompanying drawings, in which Figure 1 illustrates inend elevation one construction of magneto with extension pieces or tipsattached -to the stator pole shoes, while the rotor pole shoes areformed with slots the rotor beii' oshown in the telly advanced tuningposition.

Figure 2 is an illustration of the same n'r gne'toasin'lligure l, butwith thcrotor in a partly retarded timing position when the contactbrealier points open.

{oh ore 3 also illustrates a s' ilar mag neto tot-hat of Figure 1 but'ith the rotor in the fully retarded tiini position when thecontactbreaker points open.

Figure l illustrates a modification oil the devices showi i in Figures1., 2 and 3 in which extension pieces or tips extending less than haltthe length of the stator .pole shoes are provided both at the bottom andtop of each of the said pole shoes, and Figure 5 is an end elevation ofanother modifica ion in which the extension pieces or tips are .att ckedto the rotor pole-shoes and the slots are provided in the stator poleshoes.

Referring to Figures '1. to 3, the stator pol ,saoeaA an .B'a e each p oided on heir 'leaiiis-et ges with an extension pie or tip C connected tothe main tip oi ease-s the pole sboe'oy a bridge nieinber Doi' lowmagnetic reluctance, while the rotor pole shoes are formed withlongitudinal slots E. Z l llC tipsiC inayuextend the whole length. orthe stator pole shoes, (measured inan axial direction) or they maybeshortei than the said length; in'the former case the slots E extend thefull length or the rotor pole shoes (measured in an axial direction)while in the. latter case it is preherredto make the ots slightly longerthan the tips C in der to reduce leakage at the end of the 'l' ie slotsE are of such width and so c 1 that theylie und r the main porof thetipsC when the rotor is in the correct position for the contact breakerpoints to open with the timing fully advanced, tiis position being shownin Fig;- ure 1, where it will/be seen that there is a comparativelylarge air gap between the s C and the rotor pole shoes in radialdirection, so that the tips C exercisepractically no detrimental'eiicct,the breaking edge being then between the tips on the rotor pole shoesand the statorcpole shoes at l and also between the trailing edge of thei'nain portion of the rotor pole shoe andthe trailing edge of the statorextended tip at C. On partly retarding the timing, the rotor will havemoved further round at the n'ioinent when the-contacts open, as shown inFigure .2, so that the unslotted portions "5 the rotor now'lie unde-rthetips G, which latter have the eiiect of bridging across the air spacesat top and bottom between the main pole shoes of the stator and therotor pole shoes.

Figure 3 shows the position of the same rotor when the contacts openwith the timinc fully retarded. the flux break taking place in thisposition between the ends of the tips C and the ends of the unslottededges of the rotor pole shoes,thc effect oi the tips'hcinp; again tobridge across the air space and so to reduce its reluctance.

In the modification shown in Figure 4 both the top and bottom of eachstator pole shoe provided with extension pieces or tips C, these being:constructed. oi less than l altthc length (TllGQSlllJ-Gfl in an axialdirection) of the main stator pole shoes and being; staggered so thatthe tips attached to o 'pgositcj poles do not he too close to oneanother. This construction allows the bot 7; of the magneto to be usedboth for clockwise and counter-clockwise machines.

Lastly in the construction shown in Figure '5 the extension pieces ortips C are attached to the rotor pole shoes by the'bridaes D and theslots E are then torinedin the requisite positions in the stator poleshoes,

instead of in the rotor pole shoes as in the case of the arrangementspreviously described. With "this construction when the rotor is in theadvanced timing position the break takes place between the stator androtor pole shoes at F and also between the trailing tip of the rotor atC and the edge of the stator shoe at H, the rotor tips C being separatedfrom the stator pole shoes in a radial direction by a comparativelylarge air gap, this being the position illustrated in Figure 5. As thetiming is retarded the air gap between the rotor tips C and the statorpole shoes is reduced, until finally the break occurs between the edgesG of the stator pole shoes and the rotor tips C. By forming each of thestator pole shoes with two slots therein as illustrated, one above thehorizontal axis of the rotor and one below the said axis, the same bodymay be used for both clockwise and counter-clockwise machines.

It is preferred to construct the rotor pole shoes so that the dimensionmeasured from the axis of the rotor to the extended trailing tip C ofthe rotor pole shoe is less than the dimension to the main body of therotor pole shoe at F so as to avoid any risk of a maximum sparkingposition occurring when the rotor is in the mid-position as indicated inFigure 2.

The machines illustrated in the drawings are of the rotary magnet andstationary armature type, but it is to be understood that the inventionmay be applied to any type of magneto which is provided with variableignition. Devices constructed in accordance with the present inventionare of special applioability where it is desired to provide a largerange for the ignition timing and give a greatly improved performance inthe re tarded timing position without any dctrimental effects to thesparking characteristics in the advanced timing position beingexperienced.

lVhat we claim and desire to secure by Letters Patent of the UnitedStates is:

1. An ignition magneto of the type hereinbefore referred to comprising arotary member and a stationary member, each of the pole shoes of one ofthe aforesaid mem bers being provided Wit-h an extension piece or tipconnected therewith by a bridge member of low magnetic reluctance sothat the said extension piece is separated from the main body of thepole shoe by a recess, and each of the corresponding pole shoes of theother member having a recess or slot therein separating the main body ofthe pole shoe from the end of the tip-like portion thereof,

the total circumferential length (measured in the plane of rotation) ofthe extension piece and recess of each pole shoe of one member beingsubstantially equal to that of the recess and tip of each pole shoe ofthe corresponding member, and the extension piece and recess of eachpole shoe of one member being so located with regard to the recess andtip of each pole shoe of the corresponding member that for the advancedtiming position of the rotor the extension pieces of the pole shoes ofone member lie opposite the recesses or slots of the pole shoes of theother member and are separated therefrom in a radial direction by acomparatively large air gap when the flux break occurs, there being twopoints of flux break on each side of the rotor in the said advancedtiming position, whilst for the retarded timing position of the rotorthere is only one point of flux break on each. side of the rotor, thesaid flux break then taking place between the extension pieces of thepole shoes of one member and the opposite end portions or tips of thepole shoes of the other member.

2. An ignition magneto as claimed in claim 1, in which the distance fromthe axis of rotation of the rotor to the face of the trailing tip oneach rotor pole shoe is appreciably less than the distance from the saidaxis to the face of the main body of the said pole shoes.

3. An ignition magneto as claimed in claim 1, in which each stator poleshoe is provided on its leaving edge with an extension piece or tipconnected to the main body of the pole shoe in question by a bridgemember of low magnetic reluctance, said extension piece or tip being ofshorter length (measured in an axial direction) than that of the statorpole shoes, and each rotor pole shoe is formed with a recess or slottherein, said recess or slot being slightly longer than the said tips soas to reduce leakage at the end of the tips.

4. An ignition magneto as claimed in claim 1, in which the rotor poleshoes are formed with recesses or slots therein and the stator poleshoes are provided both at the top and bottom thereof with extensionpieces or tips, the said tips being less than half the axial length ofthe main stator pole shoes and staggered with regard to one another forthe purpose specified.

JAMES COCHR-ANE HUTTON. HAROLD ERNEST lVHITEHOUSlG.

